The use of microwave for communication has large capacity, good quality and can be transmitted to a long distance, so it is an important communication means of the national communication network, and is also generally applicable to various special communication networks.
China's microwave communication is widely used in the L, S, C, X frequency bands, and the application of the K frequency band is still under development.
Due to the extremely high frequency and short wavelength of microwaves, the propagation characteristics in the air are similar to light waves, that is, straight forward, and they are reflected or blocked when they encounter a block. Line-of-sight needs to be relayed forward.
Generally speaking, due to the influence of the curved surface of the earth and the loss of space transmission, a relay station needs to be installed every 50 kilometers or so to extend and extend the radio waves. This communication method, also known as microwave relay communication or microwave relay communication, long-distance microwave communication trunks can be transmitted to thousands of kilometers through dozens of relays and still maintain high communication quality.
The equipment of the microwave station includes antennas, transceivers, modulators, multiplexing equipment and power supply equipment, automatic control equipment, etc. In order to gather the radio waves into a beam and send it to a distant place, parabolic antennas are generally used, and its focusing effect can greatly increase the transmission distance. Multiple transceivers can use an antenna without interference with each other. The current microwave system in China can work simultaneously with six receptions and six transmissions in the same direction in the same frequency band, and can work simultaneously with eight receptions and eight transmissions to increase the overall capacity of the microwave circuit. . Multiplexed equipment is divided into analog and digital. Each transceiver of the analog microwave system can work in 60-channel, 960-channel, 1800-channel or 2700-channel communication, and can be used in microwave circuits with different capacity levels. Digital microwave system uses digital multiplexing equipment to form a primary group based on the principle of time division multiplexing of 30 telephones, and then can form a secondary group of 120 channels, a tertiary group of 480 channels, and a fourth group of 1920 channels, which are modulated on the transmitter by a digital modulator At the receiving end, it is restored to a multi-channel phone by a digital demodulator. The latest microwave communication equipment, its digital series standard is completely consistent with the synchronous digital series (SDH) of optical fiber communication, called SDH microwave. This new microwave device can transmit more than 30,000 digital telephone circuits (2.4Gbit / s) simultaneously with eight beam waves on a circuit.
Because of its wide frequency bandwidth and large capacity, microwave communication can be used for transmission of various telecommunications services, such as telephone, telegraph, data, fax, and color television, etc., can be transmitted through microwave circuits. Microwave communication has good disaster resistance, and it is generally unaffected by natural disasters such as floods, wind disasters, and earthquakes. However, microwave transmission over the air is susceptible to interference. The same frequency cannot be used in the same direction on the same microwave circuit. Therefore, the microwave circuit must be constructed under the strict management of the radio management department. In addition, due to the characteristics of microwave linear propagation, high-rise buildings cannot be blocked in the direction of the radio wave beams, so the urban planning department should consider the planning of urban space microwave channels so that it will not be blocked by high-rise buildings and affect communication.
In recent years, China has successfully developed a point-to-multipoint microwave communication system. The central station uses an omnidirectional antenna to transmit to the surroundings. Within 50 kilometers of the surroundings, there can be multiple points to place the user station, and then multiple telephones are branched out from the user station to answer To each user. Its overall capacity has equipment of different capacity such as 100 lines, 500 lines and 1000 lines. Each user station can allocate dozens or dozens of telephone users, and can also be extended to users hundreds of kilometers away through relay stations when necessary. . This point-to-multipoint microwave communication system is also very economical for users in suburban areas, county towns to rural villages or coastal islands, and for scattered residential areas.
Microwave communication also includes "tropospheric scatter communication" and "meteor trail communication", etc., which use the unevenness of the upper atmosphere or the scattering effect of the meteor trail on the radio wave to achieve communication beyond the line of sight. These systems are used in China. less.
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